packaging and maintaining a twin turbo charged V10 engine into the relatively cramped mid engine layout of the R8 has proved to be a rather difficult thing to manage.
as sh@un said, 3(...perhaps 4) twin turbo prototypes have done thier best impression of a roman candle in testing. the most famous of which was at the 'ring, pretty much melted everything but the basic chassis... and keep in mind that aluminium has a melting point of about 660 degrees
current plans are to use a naturally aspirated 5.2-5.5 litre V10 producing somewhere in the region of 373kw, it should be easier to keep this set up cool.